Magnetron and control



ATTORNEY IN VE NT OR J. a. F/SK 5.1

Sheets-Sheet 1 J. B. FISK MAGNETRON AND 'CONTROL Filed NOV 2, 1942 Feb.25, 1947.

Febzzs, 1947.

,1. B. FISK MAGNETRON AND CONTROL 2 Sheets-Sheet 2 Filed NOV. 2, 1942 nM T/ W M W B A Jr Z Patented Feb. 25, 1947 2,416,298 MAGNETRON AND comm.

James B. Fisk, Madison, N. J., assignor to Bell Telephone Laboratories,Incorporated, New York, N. Y., a corporation of New York ApplicationNovember 2, 1942, Serial No. 464,219

This invention relates to magnetron devices and particularly tomagnetrons for the delivery of large amounts of high frequency power.

An object of the invention is to provide improved means for controllingthe amount of power delivered by a magnetron.

Another object is to provide improved means for modulating the powerdelivered by a magnetron in accordance with a signal.

A related object is to effect-such power control or modulation, as thecase may be, without substantial departure from optimum efliciencyconditions for oscillation.

Another related object is to effect modulation of magnetron oscillationsby pure voltage control, the required power being negligible.

Still another object is to protect the active surface of a magnetroncathode from bombardment by electrons and therefore from deterioration.

Another object is to provide a magnetron which is characterized byimproved geometrical interelectrode relations.

These and other objects are attained in accordance with the invention bythe provision of a cathode of novel form and arrangement, the sensitizedsurf-aces of which are shielded from the influence of the high frequencyfields in the interaction space and therefore from bombardment byelectrons, while a control electrode is provided whose juxtapositionwith the cathode is so correlated with the electric and magnetic fieldsthat it draws electrons from the cathode substantially in the quantitiesrequired for delivery to an external utilization circuit of a desiredamount of power, and this without substantial bombardment of the controlelectrode by elsetrons or dissipation of power therein. Variation of thecontrol electrode potential, in accordance with a modulating signal orotherwise, as desired, varies the numbers of the available electronswithout in any way altering the conditions in the interaction space andtherefore without affecting the electron orbits therein or the frequencyof the resulting oscillations or the efficiency of the device as agenerator of such oscillations.

In a preferred embodiment the cathode may comprise a plurality of spacedstrips arranged in a circular row within the interaction space anddefining a central discharge space, while a control electrode may bemounted centrally in the discharge space. The cathode strips may betreated to render them thermionically emissive only on their innersurfaces, being shielded on their outer surfaces from the influences ofthe fields in the interaction space.

Inthe design of high frequency, high energy magnetrons, it isapplicant's practice to select the inside diameter of the interactionspace, the number of anode surfaces, the constants of the tuning means,i. e., in a magnetron of the solid anode cavity tuned type, the sizesand shapes of the tuning cavities and the channels or slots whichconnectthe same with the interaction space, the diameter of the centralcathode and the operating anode voltage and the relations between all ofthese quantities from considerations of eiiiciency and of theoscillation mode in which the device is intended to operate. When thesefactors have been so selected there is in devices of conventionalconstruction no satisfactory way of controlling the cathode current andtherefore the power. Cathode temperature control is inefficient at bestand, .on account of thermal inertia effects, it is out of the questionfor rapid control such as is employed in modulation. 'The use of acontrol grid surrounding the central cathode has proved unsatisfactoryas a control means for a number of reasons. First, it inevitably draws aconsidera-ble current so that the cathode grid circuit presents a lowimpedance to the input circuit from which it is supplied and theapparatus draws substantial amounts of power at the modulationfrequency. Second, to cut oil the cathode current completely, such acontrol grid must be driven to a high negative voltage. In such case thegridanode voltage might be so great as to cause excessive secondaryemission either from the grid or from the anode or even a disruptivedischarge.

Furthermore, the interaction space is defined the control electrodepotential affect only the.

number of electrons supplied from the discharge space to the interactionspace, so that neither oscillation frequency nor efllciency is greatlyaffected during the course of the modulation cycle.

The invention will be fuliyunderstood from the following detaileddescription of a preferred illustrative embodimentthereof taken inconjunction with the appended drawings, in which:

Fig. 1 is a broken perspective view of a mag- Fig. 2 is a verticalcross-section of a part of the cathode structure of Fig. l;

i netron device of the solid anode cavity-tuned 1 i type provided withthe novel cathode and control electrode of the invention;

Fig. 3 is a horizontal cross-section of the same 1 part of the cathodestructure of Fig. 1;

Fig. 4 is a broken perspective view of an alter- 3 native cathodestructure;

Fig. 5 is a simplified diagrammatic cross-section of the central portionof the magnetron of j Fig. 6 is a simplified schematic view of a mag-Fig. 1, broken in two, to show the electron paths 1 j for two diiferentvalues of control electrode po- I tential; and

3 netron of the conventionalsplit anode type, mod f ified by theaddition of the novel cathode and" control electrode.

Referring now to Figs. 1, 2an'd 3, the body of 1 the magnetron maycomprise a comparatively 3 massive block ill of conductive material,such as 1 copper, into which are cut as by drilling a cen- 1 tralinteraction space l2 and a plurality of res- -i onant cavities I4surrounding the same and symmetrically arranged about it. Each of thecavities surfaces.

The anode block I0 is preferably mounted ceneither case the shell may beclosed at the ends by plates 22 which serve both to exclude air andgases and to define the end spaces 24 in which 1 l4 opens onto theinteraction space l2 through 1 a channel or slot I'li which serves as acoupling 1 means between the energy of movement of the j electrons inthe interaction space'and the electromagnetic field within the cavity.The cylindrical f surfaces l8 between channels l6 serve as anode trallyin a cylindrical shell or casin 20 of con- 1 ductive material such ascopper, and connected 1 thereto. If preferred, anode block I 0 and shell1 20 may be machined from a single solid mass. In

the mutual flux 26 common to adjacent cavities I l4 exists.

The cathode may consist of a plurality of flat- The width of each stripmay ing between the strips. If desired, they may be slightly arched fromside to side, as indicated Each of these cathode strips 30 may be ofresistive material so that its temperature may be raised to the emissionpoint by the passage of current therethrough and their inner surfaces 32Cathode heating cur- 1 in Fig. 3, to conform with the circle on which i3 they are disposed, though fiat strips serve substantially as well,especially if their number is I fairly large.

j which face the axis of the device may be treated with a, suitablethermionically emissive material. They may be mounted as by welding ateach end i to rings 34, 36 of conductive material. End discs j 38 maybeconnected to the cathode supporting 1 rings 34, 36 extending outwardlytherefrom in a manner partly to close the interaction space l2 1 andmaintain space charge conditions within 3 it at desired values and soreduce losses due to the escape of working electrons into the end 1spaces 24 of the device. 1 rent may be supplied from a suitable sourcesuch a as a battery 40 to the end discs 38 by way of suitable relativelystifi conductors 42 which thus serve both as cathode supports and asheater leads. To minimize high frequency power losses over the heaterleads 42 the latter may, if desired, be tuned as by shcrt-circuitedcoaxial lines in well-known manner.

Outside of each of the cathode strips 30' and closely adjacent theretobut insulated therefrom may be placed a conductive shielding strip 44,for exampleof metal. Each of these shielding strips 44 may be connectedeach to the cathode strip 30 which it shields or to the cathodesupportin ring 34 at a convenient point but they are preferably notconnected to the cathode strips 30 at more than one point, else theywould serve to short-circuit the cathode strips and reduce the heatingof the latter. They may be slightly wider than the cathode strips 30 andthey may be mounted and supported in position in any convenient manner.For example, they may be 'cemented throughout their lengths to thecathode strips by adhesive insulating material 46, or they may beclamped over the ends of the cathode strips, a layer of insulatingmaterial being provided at one of the clamping points. To hold theshielding strips 44 securely, in position, a band 48 may surround themat their ends removed from their connections to the cathode strips 30through the ring '34.

An alternative form and construction for the cathode assembly is shownin Fig. 4. It may consist of a tube 50 of sheet metal of a suitablematerial through the walls of which slots have beencut. In such caseupper and lower supporting and current supply rings 52, 54 are integralwith the strips 30. If this construction be preferred for both cathodestrips and shielding strips, the outer shield tube 56 may he slippedover the inner cathode tube with the two sets of slots in alignment, aslotted tube 58 of suitable insulation material being placed between.The two conductive cylinders may then be interconnected at one end as byspot welds 60 between the upper cathode supporting ring 52 and the uppershield supporting ring 62. If desired, the tubes 50, 56, 58 may befitted together prior to cutting the slots and the slots may then all becut together. In this event since it is preferred that the shield stripsbe somewhat wider than the cathode strips, the slots are preferably cutfrom the inside outward, a tapered or shouldered tool being employed forthe purpose.

Within the central discharge space l3 defined by the cathode strips 30,the auxiliary control electrode 64 of the invention may be placed. Forexample, it may be a cylinder of metal of diameter of the order ofone-half the diameter of the cathode circle and effective lengthsubstantially equal to the axial length of the cathode strips 30. It maybe supported in position by insulating bushings 65 in the end discs 38,and potential may be supplied to-it by way of a suitable conductor 68.

Operating voltage may be applied between the cathode and the anode fromany suitable source such as a a battery 66 whose positive terminal isconnected to the anode block l0 and casing 20 which, since it isexternal to the cathode and liable to be touched by the hands of anattendant, may be connected to ground. Thus a high negative voltage isapplied to the cathode. Similarly, suitable operating potential may besupplied to the control electrode 64 by way of conductors 68 connectedto a source, for example, a modulating source III. A magnetic fieldwhose direction is axial of the device may be supplied from any suitablesource, such as a coil 14 carrying a steady current.

In operation both the cathode and heater leads 42 and the auxiliaryelectrode potential supply lead 68 will be maintained at potentialswhich are highly negative with respect to theanode block In and the endplates 22 which define the end spaces 24 through which these leads reachthe electrodes. To avoid asymmetry of the electromagnetic fields withinthe end spaces 24 due to the presence of these low potential cathode andcontrol electrode leads, and also to prevent high frequency inductiontherein and consequent power loss, these leads are preferably broughtinto the end spaces 24 in the plane of the axis of one of the tuningcavities l4. Thus, the mutual flux lines 26 emerging from this cavitypass to each side of the electrode supply'leads in such a way that onlya negligible quantity of this flux links the leads. By this expedientcoupling between the leads and the electromagnetic field'in the endspaces 24 may be reduced to a negligible value.

Power may be abstracted by way of a loop which links only the fiux 26which is mutual to two adjacent cavities I4. A loop of convenient andsuitable form may comprise a rod 16 which extends radially inward fromthe outside through the casing wall 20 into the end space 24 of themagnetron and there bends over to make contact with the end face of theanode block I0. Preferably, its course lies in a plane midway betweentwo adjacent cavities l4. Power withdrawn by way of this loop may be ledover any suitable transmission path, for example, a coaxial line, to asuitable load or utilization circuit schematically indicated by theresistor 80. If desired, a plurality of such coupling loops may beemployed, all feeding a common load, suitable phase shifting devicesbeing interposed in series with the lines connected to some of them inorder that current supplied by these lines to the common load may all bein cumulative phase relation at the output ends thereof.

Fig. is a simplified diagrammatic end view of the magnetron of Fig. 1,in which the end discs 38, the cathode and heater supporting and supplyleads 42 and the control electrode supply lead 68 have been omitted inthe interests of simplicity. In the figure the body of the magnetron hasbeen broken along a diameter to show the orbits of electrons all ofwhich start their courses from the inner surface of one of the cathodestrips 30, having been withdrawn therefrom. by the acceleratingpotential on the control electrode 64, for different values of thepotential applied thereto. This figure illustrates the geometricalrelations which are preferred for optimum efflciency of operation. Ithas been discovered that best results are obtained when the radial widthof the interaction space l2, 1. e., the distance from the circle ofshield strips 44 to the anode surfaces The number and angulardisposition of the cathode strips 30 and the shielding strips 44 is notcritical, nor is the diameter of the control electrode. It is preferred,however, that the width of the apertures separating adjacent strips besubstantially equal to the widths of the strips themselves, and that thecontrol electrode 64 be of such diameter that the radial width of thedischarge space I3, measured from the surface of the control electrodeto the inner sensitized surface 32 of the nearest cathode strip 30, besomewhat in excess of the strip width.

To obtain the best results it is of course necessary to select correctlythe cathode-to-anode voltage and the strength of the axial magneticfield in a manner to cooperate with the preferred geometrical relationsstated above to give operation at maximum efficiency at a givenwavelength.

The geometrical relations hereinabove described, while preferred, arestill not critical, and are in no sense essential to the practice of theinvention in its other aspects. For this reason no attempt has been madeto bring Figs. 1 and 6 into conformity with these preferred relations.

Coming now to the mode of operation of the apparatus, and referring toFig. 5, if the control electrode potential is negative with respect tothe cathode, the electric field at the surface of the cathode strips isnegative and substantially no electrons are withdrawn therefrom. Thiscondition represents the current cut-off condition for the magnetron. Asthe control electrode potential is raised to a small positive value, afew electrons are drawn from the inner surfaces 32 of the cathode strips30 and into the discharge space I3 as shown in the upper half of thefigure. Their movements are at low velocity and under the combinedinfluence of this comparatively weak electric field and the axialmagnetic field, these electrons turn through circular orbits 16 of shortradius, most of them returning to the inner surface of the cathodestrips at or near their points of origin. Inasmuch as the dischargespace I I is substantially shielded from the high frequency fields whichexist in the interaction space 12, these electrons return to the cathodesurfaces 32 at substantially zero velocity and therefore do not injureit by bombardment. A few of them, especially those which originate nearone side of a cathode strip 30 and close to the neighboring aperture,emerge through the latter and into the interaction space l2 defined bythe cathode circle and the anode surface l8 as indicated at 18, there toundergo their orbital movements under the combined influence of theanode voltage and the axial magnetic field. Some of these,electronsemerge through the interstices between adjacent cathode strips30 when the phases of the high frequency electro-magnetic field existingwithin the interaction space l2, and particularly in the neighborhood ofthe anode surfaces [8, are such that the electrons travel through theirpredestined orbits 18 to strike one of the anode surfaces l8. Theseelectrons contribute energy to the high frequency electromagnetic field.Others of these electronswill emerge between the cathode strips 30 intothe interaction space i2 at instants when the high frequency fields arein such phase relation that they will not reach the anode surfaces l8but will return toward the cathode at high velocity. These electrons arethose which have withdrawn energy from the high frequencyelectromagneticfield. Their paths are roughly indicated in the figure by the path 88.The return of these electrons toward thecathode and, in the apparatus ofthe invention, their return to the shielding strips 44 constitutes asource of unavoidable power losses. In conventional devices,

the bombardment of the cathode surface by the high energy returningelectrons may constitute,

3 in addition, a source of deterioration of the oath-I ode surface. Withthe construction of the invention. however, the returning electronsstrlke the shielding surfaces 44 instead of the cathode sur-:

faces 32 and therefore, while they are responsible for a part of thelosses inherent in magnetron operation, they do not injure the cathode.

As the potential on the control electrode 54 is progressively increased,more and more electrons are drawn from the inner surfaces 32 of theoathode strips 30 into the discharge space l3 wherein they travelthrough curved paths of progressively the inner cathode surface isreduced. The lower half of Fig. 5 illustrates the conditions for acontrol electrode potential of roughly the maximum value. The conditionsfor a control electrode potential of intermediate value are intermediatethose shown in the upper and lower halves of the figure. It will beobserved that with the maximum value of control electrode potential,substantially all of the emitted electrons travel 1 through paths 82 oflarger radius and escape capture by the cathode surface and emerge fromthe discharge space l3 through the apertures be- 1 tween cathode Strips30 into the interaction space l2, there to undergo their orbitalmovements 84,

86. With proper selection of the spacing of the cathode strips 30, thepotential and diameter of the control electrode 64, and the diameter ofthe ring of cathode strips 30, it can be arranged that 1 none of theelectrons, even when emission is 1 maximum as shown in the lower half ofFig. 5, i strike the control electrode 64, but rather pass by the latterand out through the apertures in the l cathodestructure intotheinteraction space I2.

1 As the control electrode potential is progresi sively raised from zeroto the highest positive a value, more and more electrons emerge through1 the cathode apertures for two reasons: first, bei 3 cause the supplyof electrons from the cathode surfaces 32 to the discharge space I3 isincreased as the accelerating field at the cathode surfaces from theinner cathode surfaces 32 emerges I to an otherwise conventionalmagnetron of the split-anode type, of which alternate anode segf forexample, an antiresonant circuit 90 consistthrough the apertures intothe interaction space i I 2 to undergo their predestined orbits. Whenthe supply of electrons is a maximum, as shown in the lower half of Fig.5, a virtual cathode may be formed in a circle coincident with or nearto the cathode circle. By reason of the preferred equality between thearc lengths of the cathode strips 30 and the spaces between them, at themaximum value of the control electrode potential, substantially all ofthe electrons emitted from any one The invention is not limited in itsapplication 1 to magnetrons of the solid anode cavity-tuned type, butmay be applied to magnetrons of other types as well.

Fig. 6 is a simplified schematic plan view of the novel cathode strips30 and the 1 control electrode 64 of the invention as applied ing of aninductance element and a capacitance element'connected in parallel. Theelectrodes may be mounted in any suitable manner within an evacuatedenvelope 92, for example, of glass, while the tuning elements may bemounted within the same envelope or externally thereto as desired. Anarray of cathode strips 30 and shielding strips 44, which may beconstructed as described in connection with Figs. 1 to 4, a controlelectrode 64, and a magnetizing coil 14 are indicated. Cathode andheater leads as well as anode and control electrode potential supplyleads may be brought into the envelope as through air-tight seals inwell-known manner. Such'structural details have been omitted from thefigure in the interests of simplicity.

The novel cathode and control electrode arrangement maybe employedseparately or together as desired. They are by no means limited tomagnetrons in which the anode surfaces are circularly arranged, but maybe applied as well to magnetrons in which the anode surfaces arelinearly extended, while the novel geometrical relations apply equallyto a circularly arranged magnetron having a central cylindrical cathodeand no internal control electrode.

Still other variations and modifications of the illustrations shown anddescribed herein, which may be made without departing from the spirit ofthe invention, will suggest themselves to those skilled in the art.

What is claimed is:

1. A magnetron device comprising a hollow apertured cathode memberthermfonically emissive on its inner surface but not on its outersurface, an anode mounted opposite the outer surface of said cathodemember, a control electrode disposed within said hollow cathode member,and magnetic means adjacent said cathode for establishing a magneticfield axially of said device, whereby electrons which are drawn fromsaid inner cathode surface move in curved paths past said controlelectrode and pass out through the apertures of said cathode elementinto the space between said cathode element and said anode.

2. A magnetron device comprising an array of cathode members spacedapart in a closed configuration defining a discharge space, an anodemounted outside said closed configuration of cathode elements andopposite thereto, magnetic means adjacent said array of cathode membersfor establishing a magnetic field axially of said device for causingelectrons originating at a cathode member to travel over curved paths tosaid anode, and a control electrode axially disposed.

within said closed configuration of cathode elements to control theamount of electron current flowing from said cathode members.

3. A magnetron device which comprises a plurality of anode surfacesurrounding an interaction space, a hollow apertured cathode membermounted axially of said interaction space, which cathode member isthermionically emissive on its inner surface but not on its outersurface, a control electrode disposed within said hollow cathode member,and means for establishing a magnetic field axially of said device,whereby electrons which are drawn from said inner cathode surfaces movein curved paths past said control auaaes electrode and pass out throughsaid cathode apertures into said interaction-space.

4. A magnetron device which comprises a plurality'of anode surfacesdefining an interaction space, an array of cathode members spaced apartin a closed configuration and defining a discharge space, means forestablishing a magnetic field axially of said device for causingelectrons originating at said cathode to travel over curved paths tosaid anode, and a control electrode axially disposed within said closedconfiguration of of said shielding strips being slightly in excess ofthe width of the cathode strip to which it is adiacent.

6. A cathode structure for a magnetron device which comprises a firsthollow cylindrical member provided with a number of spaced slotspiercing the wall thereof, the inner surfaces of the unpierced portionsof the wall of said cylinder being thermionically emissive, a secondhollow cylindrical member provided with a like number of spaced slotspiercing the wall thereof, said second cylinder surrounding said firstcylinder with the slots of the two in alignment, the slots of saidsecond cylinder being slightly narrower than the slots of said firstcylinder.

7. A discharge device which comprises a plurality of anode surfacescircularly mounted about an axis of symmetry and defining an interactionspace, an array of conductive shielding members circularly mountedwithin said interaction space, and a source of electrons within thecircle of said shielding members, the radial distance from saidshielding members to said anode surfaces being of the order offour-fifths of the circumferential distance separating correspondingpoints of said anode surfaces.

8. A magnetron device comprising an anode structure defining asubstantially cylindrical interaction space, a cathode comprising aplurality of strips disposed within said interaction space on-thecircumference of a circle coaxial with said interaction space and spacedapart by distances substantially equal to their widths, said stripsdefining a discharge space within said circle and said strips beingthermionically emissive on their inner surfaces but not on their outersurfaces, 9. control electrode disposed on the axis of said dischargespace, said control electrode adapted to have a positive potential withrespect to said cathode to accelerate electrons in the direction awayfrom the inner surfaces of said cathode strips and towards said controlelectrode, means adjacent said cathode for establishing a magnetic fieldaxially of said device to constrain electrons to follow curved orbitssome of which return to the same strip from which the electron isemitted and others of which pass between strips into said interactionspace and means connected to said control electrode for varying thepotential of the controlelectro'de to control the amount of electroncurrent passing between strips.

' JAMES B. FISK.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,063,342 Samuel Dec. 8, 19362,324,776 Hergenrother July 20, 1943 1,991,632 Scofield Feb. 19,19351,980,804 Koch Nov. 13, 1934 2,217,745 Hansel! Oct. 15, 1940 2,018,314Nyman Oct. 22,1935 1,560,183 McCullough Nov. 3, 1925 1,975,610 Koch Oct.2,1934 2,091,439 Farrisworth A118. 31, 1937 1,751,418 Paul Mar. 18, 19301,714,406 Smith May 21, 1929

